1. Field of Invention
The present invention relates to a motor, and more particularly to a high-speed rotating motor for uses as a rotating apparatus for a rotative polygonal mirror or the like.
2. Related Art
The motor for use as an apparatus for rotating a rotative polygonal mirror has a rotor case on which the rotative polygonal mirror is mounted, a frame for rotatively supporting the rotor case, a rotor magnet fixed to the rotor case through a yoke and an armature fixed to a frame such that the armature is positioned opposite to the rotor magnet.
The rotor case has a cylindrical portion which creates a space in which the armature is disposed. The cylindrical yoke is fixed to the inner surface of the cylindrical portion, while the cylindrical rotor magnet is fixed to the inner surface of the yoke. When the yoke and the rotor magnet are fixed to each other, the conventional method is performed such that the overall outer surface of the yoke is bonded to the inner surface of the cylindrical portion. Moreover, the overall outer surface of the rotor magnet is bonded to the inner surface of the yoke.
The rotor case is usually made of aluminum alloy having excellent machinability. On the other hand, in general, the yoke is manufactured by pressing a thin rolled plate and the rotor magnet is manufactured by sintering.
The motor structured as described above has the rotor case made of aluminum alloy and the yoke made of steel. Therefore, the rotor case has a thermal expansion coefficient larger than that of the yoke. It leads to a fact that the cylindrical portion of the rotor case greatly expands in radial direction as compared with the yoke when the ambient temperature has been raised. Since the conventional motor has the structure that the overall outer surface of the cylindrical yoke is bonded to the inner surface of the cylindrical portion, smooth expansion of the cylindrical portion of the rotor case is, however, prevented by the yoke. As a result, distortion of the cylindrical portion occurs. Thus, there arises a problem in that imbalance of the rotor case occurs.
The rotor magnet has a thermal expansion coefficient which is considerably smaller than that of the yoke. Therefore, when the yoke is expanded by heat, the rotor magnet cannot follow the expansion of the yoke. However, the conventional motor has the structure that the overall outer surface of the cylindrical rotor magnet is bonded to the inner surface of the cylindrical yoke. Therefore, when the yoke has been expanded, great force is exerted on the rotor magnet. Thus, there arises a problem in that brittle fracture of the rotor magnet occurs. When the cylindrical portion of the rotor case and the yoke have been deformed outwards in the radial direction during high speed rotation of the rotor case, the rotor magnet cannot be deformed by centrifugal force exerted on the rotor magnet. The reason for this is that the rotor magnet is a brittle member molded by sintering or the like. Since the conventional motor has the structure that the overall-outer surface of the cylindrical rotor magnet is bonded to the inner surface of the cylindrical yoke, the rotor magnet is pulled outwards in the radial direction through the yoke. Thus, there is a probability that brittle fracture occurs.